This invention generally relates to apparatus for forming selvages in weaving machines or looms and more particularly to the detection of unfavourable conditions relating to the selvage threads in the selvage forming apparatus.
Particularly in shuttleless looms, after each picking motion, the weft thread is cut near the ends of the woven cloth and therefore the cut ends of the weft thread are apt to enter the inside of the cloth if they shrink. More specifically, some warp threads near the cloth ends will become loose even when only a slight force is externally applied thereto. This is also applicable to a wide cloth which is cut at the center during the weaving. To remove the above-discussed disadvantages, a selvage forming apparatus is generally disposed near the endmost ground warp thread forming the cloth so that a selvage is provided to the cloth by causing the weft threads to be caught by selvage threads. In general, such a selvage forming apparatus is of the type including a shed forming member with selvage thread guide eyelets, through which two selvage threads are guided. Upon rotation of the shed forming member, the two selvage threads make downward and upward movements to form therebetween a shed into which the weft thread is inserted. The inserted weft thread is intertwined with the selvage threads during the next downward and upward movements thereof, thus forming the selvage.
As can be understood, to some extent a tension is always applied to each selvage thread passing through the shed forming member to the fell of the cloth, and the machine operator may encounter abnormal conditions, such as excessively decreased tension and breakage of the selvage thread. To detect such abnormal conditions, various devices have been heretofore provided. For example, each of Japanese Laid-Open U.M. Specification No. 53-90667 and U.S. Pat. No. 4,166,480 shows an abnormal condition detecting device of the type including a pivotable arm disposed in the immediate vicinity of a selvage thread bobbin and supporting a portion of the selvage thread at its free end to detect the tension of the selvage thread. In this type of detecting device, the free end of the arm is adapted to turn a small amount upon the occurrence of a change in the tension of the selvage thread and when the arm turns a large amount due to breakage of the selvage thread, the free end thereof approaches a proximity switch to thereby turn on the same, thus detecting the abnormal condition of the selvage thread.
However, such a detecting device is not only a complex mechanism necessarily including, in addition to the pivotably supported arm, a spring for biasing the arm into an operative, i.e., abnormal condition detecting, position against the tension of the selvage thread, said proximity switch operative in response to the turning of the arm, but also requires a sufficient space near the selvage thread bobbin to contain these elements. Furthermore, since this detecting device employs a proximity switch, in order to assure the turning on of the proximity switch when an abnormal condition occurs the operating element attached to the free end of the arm and operatively associated with the proximity switch must always be moved into a fixed position with respect to the stationary proximity switch. However, it is very difficult for manufacturing reasons to mount the arm and the proximity switch so that the said operating element can always be moved to the fixed position.
It is therefore a principal object of this invention to provide a device for detecting abnormal conditions of selvage threads which is simple in its construction and installation, does not require a large mounting space, and carries out certain detection of the abnormal conditions of the selvage threads.